1. Field of the Invention
The present invention relates to a cold folding method for a hollow tube or pipe which is made substantially of magnesium, such as square tube, spectacle-type tube, circular tube, etc., wherein the tubes are folded by a press-working technique.
2. Prior Art Technique
When various kinds of press-working products manufactured from square tubes of iron are made into scrappage, and recycling of the scrappage is encouraged for resource preservation purposes, there are serious problems and disadvantages in that recycling requires a huge cost for recovery operations and transportation because the iron products are heavy and likely to be rusted and corroded and, therefore, provide serious problems and difficulties in handling and management. Moreover, the benefit gained by the resource recovery is quite small and it is extremely difficult to attempt to raise a recovery rate of the resources to thereby reduce the volume of waste.
Magnesium material and its products, which are generally substituted for iron material and products, being less efficient in recycling effect are light-weight, have a specific gravity of 1.74, which is about one-fourth of that of iron, and have a desirable handleability. Besides, the magnesium material, if molded by repeated press-working, provides a higher strength, which is as high as that of iron-made pipe, due to work-hardening. Therefore, keen attention has been paid to usage and application of the magnesium material as a metal material having a high recycling effect.
Magnesium material, which has less viscosity at room temperature, provides less extensibility of material when it is treated by a cold-press working and therefore, has some difficulty in a uniform extension of the material. Thus, a non-uniform pressure added to the material will result in the occurrence of cracks. Further, since it has a strong repulsive force, that is, a spring-back force against bending or folding by the press-working, it is difficult to place the material into a predetermined stable form and shape. The cold-press working requires a highly skilled technique, as well as further time and labor and, therefore, it is noted that folding work for a square tube is extremely difficult. Thus, in the conventional method, a hollow tube was filled with some padding before the folding work is conducted and this conventional method has serious problems of being time-consuming and inefficient workability.
Strength of a spectacle-type tube which is made of a magnesium material is examined by the help of Prof. Utsuo Yarita, of the Mechanical Engineering Dept., Chiba Institute of Technology, Chiba prefecture, Japan. The spectacle-type tube which was examined had a length of 500 mm, a diameter of 28 xcfx86 and a weight of 2.5 t, with its opposed ends being supported and a central load of 300 kgf added to obtain measurement of flexure under the conditions described above.
Flexure mm: Y; Load: P=300 kgf; Diameter of the tube: 28 xcfx86;
Length of the tube: L=500 mm; Geometrical moment of inertia: I;
Modulus of longitudinal elasticity (Young""s modulus);
E=4500 kg/mm2; Y=xe2x88x92PL3/48EI
Comparing the tube having a diameter of 28 xcfx86 with the spectacle-type tube described above, it was found that a flexural rigidity of the spectacle-type tube was larger in strength, as large as 23 times in a vertical direction and 4.4 times in a lateral direction. The experiment shows that the spectacle-type tube has an excellent mechanical strength.
In view of the foregoing, it is an object of the present invention to provide a new cold folding method for hollow tubes, made substantially of magnesium material, such as square tubes, spectacle-type tubes, general circular tubes, wherein the magnesium material is the lightest in weight among the structural metal materials and has an excellent recycling property and a resource-recovery property, and has energy-saving properties having environmental suitability.
According to a first aspect of the present invention, there is provided a cold folding method for a hollow tube of magnesium material, wherein the method uses an apparatus comprising:
a male mold, movable up and down, having a male lower surface portion at a lower portion thereof, a male side surface portion at opposite sides thereof, a male curved surface portion between the side ends of the male lower surface portion and the side surface portion, a lower protection member at the male lower surface portion and side portion protection members at the opposed sides of the side surface portions,
female molds, in a paired configuration, each having a female front surface portion, a female upper surface portion, a female curved surface portion between the female front surface portion and the female upper surface portion, a front portion guide member at the front surface portion, and an upper portion guide member at the upper surface, the front portion guide member and the upper portion guide member being located at the lower position of the male mold in a spaced confronting relation with each other,
wherein the method comprising the steps of:
providing an inner rib at opposite sides of a hollow square tube made of magnesium material,
accommodating a middle portion of the square tube, having opposed ends received in the upper portion guide members of the female molds, in an inner portion of the lower portion protection member of the male mold, and
lowering the male mold to fold the square tube by the combination of the curved surface portion of the male mold and the curved surfaces of the female molds.
In a preferred mode of the invention described above, a curved surface of the curved surface portion of the male mold is as large as 4.5 times or more of a curved surface of a folding diameter of the square tube.
In another preferred mode of the invention described above, the inner ribs are positioned, in a depressed configuration, on the opposite sides of the folded portion of the square tube such that each of the inner ribs is longer than a length of the curved surface.
In a further preferred mode of the invention, the lower portion protection member on both of the sides of the lower surface portion of the male mold and the upper portion guide member on both of the sides of the upper surface portions of the female molds are formed to accommodate the square tube therein.
According to the first aspect of the invention described above, while the square tube is protectively held at its opposed side surface portions by the lower portion of the protection members on the opposed sides of the male lower surface portion of the male mold and the upper guide member on the opposed sides of the female upper surface portion of the female molds which are movable toward and away from each other, the tube is pulled in and folded by the male curved surface portion and the female curved surface portions. Accordingly, the inner ribs are formed in the form of recesses on the opposed side surfaces of the folded portion of the tube, so that a mechanical strength is increased by raised portions which are formed at the circumferential portions of the inner ribs to thereby prevent the generation of cracks.
According to a second aspect of the present invention, there is provided a cold folding method for a hollow tube of magnesium material, wherein the method uses an apparatus comprising:
a male mold, movable up and down, having a recessed surface portion having a recess portion of the same diameter as a radius of the tube, a male lower surface portion, a male side surface portion on the both sides of the male lower surface portion, a male curved surface portion connecting the male lower surface portion and the male side surface portion, the male recess surface portion being formed continuously in the male lower surface portion, the male side surface portion and the male curved surface portion, and a male abutting member on the opposite sides of the male lower surface portion, the male side surface portion and the male curved surface portion,
female molds of a paired configuration, movable back and forth, each having a female front surface recess portion having a recess of the same diameter as a radius of the tube on a front surface, a female upper surface recess portion having a recess of the same diameter as a radius of the tube on an upper surface, a female curved surface portion on a coupling portion between the front surface and the upper surface, a female curved surface portion on a coupling portion between the female front surface recess portion and the female upper surface recess portion, a female abutment portion formed continuously on the opposed sides of each of the female front surface recess portion, the female curved surface portion and the female upper surface recess portion,
wherein the method comprising the steps of:
accommodating, inside the male recess surface portion of the male mold and the female curved surface portion of the female molds, the opposed ends of the hollow tube of a magnesium material into the female upper surface recess portion of the female molds, and
lowering the male mold to fold the tube by the combination of the male curved surface portion on the lower surface of the male mold and the female curved surface portion of the female mold.
In a preferred mode of the invention of the second aspect described above, the male abutment portion is formed on the opposed sides of the male recess surface portion on the lower surface of the male mold, the male recess surface portion having a recess of the same diameter as a radius of the tube on the opposed sides, and the female abutment portion abutting against the male abutment portion is formed on the opposed sides of the female recess surface portion on the front surface and the upper surface of the female mold and on the opposed sides of the female curved surface portion of the coupling portion between the front surface and the upper surface.
In a further mode of the invention of the second aspect, the curved surface of the male curved surface portion of the male mold is folded to an extent that the curved surface becomes in the range of 3.2 to 3.5 times as large as the folding diameter of the tube.
According to the present invention, the male abutment portion formed on both of the sides of the male recess surface portion is placed in an abutment relation with respect to the female abutment portion formed on both of the sides of the female curved surface portion, so that the hollow tube in the form of a spectacle or binoculars configuration (hereinafter referred to as xe2x80x9cspectacle-typexe2x80x9d tube) is protected at its outside, and the male mold is lowered while the opposed ends of the tube are received or accommodated in the upper recess portion of the female mold. Then the fold-working is carried out while the tube is being drawn by the combination of the male curved surface portion of the male mold and the female curved surface portion of the female mold. This permits a desirable and refined finish of the folded tube.